1. Field Of the Invention
This invention relates to cardiac pacing systems and, more particularly, to dual chamber pacing systems designed to react to an occurrence of retrograde conduction by switching to a different mode of operation; to detect when such retrograde conduction has ended; and to resume normal synchronous tracking directly upon detected end of retrograde conduction.
2. Description of the Prior Art
The advantages of dual chamber pacing, and more particularly pacing in different modes which are selected in response to different patient conditions, is now well recognized in the art. Early pacing systems were solely ventricular, and were sufficient for management of patients with complete heart block and Stokes-Adams attacks. However, ventricular demand pacemakers are not equipped to take advantage of atrial activity, and thus are limited in their efficiency. Subsequently, atrial synchronous, ventricular pacemakers were introduced, having a lead for sensing P signals from the atrium and another for pacing the ventricle after a suitable P-R (A-V) interval. Such a pacemaker, e.g. VDI or VDD, allows the atrium to control the heart's response rate, the ventricle being paced at the atrial rate up to a predetermined upper rate limit. Such synchronous pacers have incorporated means for dealing with high atrial rates, including "block" and "Wenckebach" technic ues.
Another form of A-V or dual chamber pacer that has been utilized is the sequential pacemaker (DVI), which paces both the atrium and the ventricle with an appropriate A-V delay which is timed by the pacemaker. A number of commercial pacemakers have been introduced which are programmable to these and other known pacing modes. Each of the various operating modes is particularly adapted to certain circumstances that may arise in a given patient.
Since the dual sense-dual pace DDD pacemaker became commercially available, it has gained favor for the reason that it compensates for many of the disadvantages of other pacemaker modes. The classic DDD pacemaker is described in U.S. Pat. No. 4,920,965, Funke et at., in some detail. See also U.S. Pat. Nos. 4,539,991 and 4,554,921, incorporated herein by reference, which disclose other forms of DDD-type pacemakers.
More recently, the DDDR pacemaker has come to prominence. In this type of pacemaker, there is provided one or more sensors which enable the pacemaker to be rate responsive, such that the pacing interval, or escape interval, is varied as a function of one or more sensed rate-indicating parameters, rather than being fixed at a programmed value. In the DDDR pacemaker, both atrial and ventricular natural beats may occur so long as they occur prior to the respective rate responsive escape interval. See U.S. Pat. Nos. 4,467,807 and 4,951,667, which are illustrative of dual chamber rate responsive pacemakers.
There have also been disclosed multi-mode pacemaker designs having means for switching modes in response to changing patient conditions. Most dual chamber pacemakers are programmable to distinct modes, or switch automatically from one mode to another under certain prescribed conditions. See, for example, U.S. Pat. Nos. 4,527,568, and 4,920,965. However, there remains a substantial need in the pacing art for sensing the conditions under which a dual chamber pacemaker can or should be controlled to change mode, and for providing optimum flexibility for blending two or more modes of operation. Thus, instead of forcing the pacer to operate in a distinct mode until patient history enables switching to another distinct mode, the pacer is optimally enabled to react on a cycle-to-cycle basis to sensed events. For example, while it is desirable to synchronize a delivered ventricular stimulus to a sensed atrial signal whenever possible, at the same time the pacemaker should be controlled to adopt another more optimum response whenever desired. Thus, if an atrial sensed beat occurs at too high a rate, or if retrograde conduction is determined, the pacemaker should have the flexibility to respond quickly and appropriately to the situation.
It is known in the pacemaker art to provide for control of pacemaker-mediated tachycardia (PMT). See U.S. Pat. No. 5,247,929, assigned to the same assignee as this invention, and incorporated herein by reference. In the pacemaker of the referenced patent, when retrograde conduction (RC) is detected, the pacemaker switches from a synchronous tracking mode into an asynchronous mode, i.e., ventricular pace pulses are delivered without regard to the timing of the atrial signals. Of course, when the pacemaker goes into such an asynchronous mode of operation, it becomes important to sense when the retrograde conduction is ended, so that the pacemaker can go back to normal synchronous tracking operation. In the pacemaker of the referenced patent, the system waits until either the occurrence of certain patterns of events that indicate the end of RC, or until the expiration of a programmed wait time following which the pacemaker goes through a routine for checking to determine the presence of RC. This routine involves an active search carried out by changing the AV or VV interval to a different, predetermined interval for a number of consecutive cycles, and monitoring of the resulting VA interval. A measure of the resulting VA interval is determined and analyzed for VA equality, i.e., substantially equal VA intervals indicate an RC condition. If such equality is not determined, the pacemaker can conclude that RC has ended. However, this procedure is subject to situations where RC had ended before the search was initiated. Alternately, the search can be conducted too early, resulting in recurrent PMTs for some duration of operation.
In view of the prior art, there remains a substantial need for a dual chamber pacing system which optimally reacts to episodes of RC by switching into an asynchronous mode of operation, and then quickly returns to synchronous tracking of atrial signals when the RC has ended. The specific need is for a system of detecting end of RC as quickly has can be reliably done, even on a cycle-by-cycle basis when appropriate.